Early Cambrian Xiate Gabbro in Western Tianshan: Magmatic Records of Initial Subduction of the South Tianshan Ocean

Compared with the widespread magmatic events between the late Paleozoic and the early Mesozoic in Western Tianshan, the Geologic record of the early Paleozoic are preserved less, which conatrained our understanding on the early evolution of the Asian Ocean. Xiate gabbro is exposed in the north of the Southern Margin of the Central Tianshan, and zircon LA-ICPMS U-Pb dating shows that the formation age is 523±5 Ma, suggesting the Early Cambrian emplacement. The geochemical characteristics of the rocks studied show that they have experienced the fractional crystallization of olivine, spinels, and the cumulation of plagioclase during their formation. The gabbro is a sodic and calc-alkaline series rocks, rich in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE). Discrimination diagrams of tectonic magmatic environment indicate that it is a product of arc magmatism. The ε Hf (t) values range from +1.47 to +11.91, indicating a distinct mantle material source; the higher (Th/Nb) _PM and lower Nb/La ratios imply the involvement of subduction materials during magmatic evolution. In combination with the research on the evolution of regional igneous rocks, we believe that the Paleozoic subduction of the Paleo-Asian Ocean initiated in the west and then gradually expanded to the east, which may lay the foundation for the scissors closure of "earlier in the west and later in the east" in the western Tianshan. Moreover, the formation age and petrogenetic characteristics of the Xiate gabbro indicate that the South Tianshan Ocean began to subduct towards the Central Tianshan Block in the Early Cambrian, and the initial arc magmatism was formed during the tectonic event.